In EP 0 023 433, there is illustrated and described a high speed ink jet printer of the rotatable drum type having a print head for printing an image on a substrate during n printing passes where b=n.times.a, "b" is the ink jet head separation gap between adjacent ink jet heads and "a" is the inter ink dot line separation gap between adjacent ink dot lines i.e. a pixel length, each printing pass being constituted by a drum revolution and the displacement of the print head along the substrate through a step equivalent to the pixel length between consecutive drum revolutions.
FIG. 1 shows an ellipse shaped image printed during three drum rotations of such an ink jet printer having 5 ink jet heads labelled A, B, C, D and E. For convenience, the ink dot lines printed by the different ink jet heads during each consecutive drum rotation are denoted by the reference numbers 1, 2 and 3. It can be readily appreciated that the image suffers from the drawback that a single defective ink jet head C having, say, a blocked nozzle, causes an unprinted band of a width equal to the inter ink jet head separation gap.
It is the object of the present invention to substantially overcome this and other disadvantages.